Cysteine derivatives, represented by the formula [I] wherein R.sup.1 and R.sup.2 are hydrogen or acyl group, are known as useful agents for the treatment of liver damage, rheumatic diseases and cataracts (for example, Japanese Kokai Tokkyo Koho No. Sho 53-5112, 54-5916, 55-51020, 55-51021, 55-51054 and 55-92315 ).
But disulfide derivatives of the oxidized form of the compound, represented by the formula [I] wherein at least one of R.sup.1 and R.sup.2 is hydrogen, are not disclosed.